1. Field of the Invention
The present invention relates to a radio IC device and a radio communication terminal, and more particularly, to a radio IC device preferably for use in an RFID (Radio Frequency Identification) system and a radio communication terminal which includes the radio IC device.
2. Description of the Related Art
In recent years, as an information management system for articles, an RFID system has been put into practical use, in which a reader/writer that generates an induction field communicates with an RFID tag (also called a radio IC device) attached to an article, using a non-contact method which utilizes an electromagnetic field, so that transmission of specific information is carried out. For example, a radio IC device mounted on a substrate of an electronic apparatus, such as a radio communication terminal, has been used for information management of the substrate and body of the electronic apparatus.
Recently, in particular, a dual-interface communication IC mounting a digital interface for wired communication, as well as an RF interface for radio communication, has been developed and attracted attention, as a radio IC chip that stores specific information and processes a specific radio signal.
In an RFID system utilized for managing a substrate of an electronic apparatus such as a portable terminal, using, as an antenna of a radio IC device, a ground conductor of a printed board built in the apparatus is described, for example, in Japanese Patent No. 4535209. The use of the ground conductor as an antenna eliminates the necessity of a dedicated antenna. Therefore, the size of the electronic apparatus may be reduced, and a gain in a radiation plate (ground conductor) that functions as an antenna may be improved.
However, in the dual-interface communication IC mentioned above, when the ground conductor of the apparatus body is also used as an antenna, the fact that the ground conductor also serves as a reference potential for the digital interface will affect communication characteristics of the RFID system (for example, the frequency characteristics vary and the reading distance decreases). Further, in the digital interface, because of the presence of a circuit which is not isolated from the ground conductor in terms of high frequencies, the communication characteristics may be affected depending on the connection status of such a circuit.